Steels used for ships, marine structures, and pressure vessels are subjected to welding and formed into structures with desired shapes. Accordingly, these steels are required not only to have high strength and excellent toughness in base metals from the viewpoint of safety of the structures, but also to have excellent toughness in welded joints (weld metals) and weld heat-affected zones (hereinafter, referred to as “HAZ”).
As the basis for evaluation of toughness of steel, the absorbed energy by the Charpy impact test has been mainly used. In recent years, in order to enhance reliability, the crack tip opening displacement test (hereinafter, referred to as the “CTOD test”) has been often used. In this test, a specimen having a fatigue precrack in a toughness-evaluating portion is subjected to three-point bending, and the amount of crack tip opening (plastic deformation volume) immediately before failure is measured to evaluate the resistance to occurrence of brittle failure.
In the CTOD test, since a fatigue precrack is used, an extremely small region can be a toughness-evaluating portion. When there is a local embrittlement area, even if good toughness is obtained by the Charpy impact test, low toughness may be shown by the CTOD test in some cases.
Local embrittlement areas are likely to occur in the weld heat-affected zone (hereinafter, also referred to as “HAZ”) which is subjected to a complicated thermal history due to multilayer welding in a thick steel plate or the like. The bond (boundary between the weld metal and the base metal) and a region in which the bond is formed into a dual-phase region by reheating (region in which coarse grains are formed in the first cycle of welding and which is heated into a ferrite and austenite dual-phase region by the subsequent welding pass, hereinafter, referred to as the “dual phase re-heating area”) correspond to local brittle areas.
Since the bond is subjected to a high temperature just below the melting point, austenite grains are coarsened and are likely to be transformed, by the subsequent cooling, into the upper bainite structure having low toughness. Thus, the matrix in itself has low toughness. Furthermore, in the bond, brittle structures, such as the Widmannstatten structure and island martensite (M-A constituent) (MA), are likely to be formed, and thereby, the toughness is further degraded.
In order to improve the weld heat-affected zone toughness, for example, a technique in which by finely dispersing TiN in a steel, coarsening of austenite grains is suppressed or TiN is used as nuclei for the ferrite transformation has been practically used. However, in the bond, heating may be performed to a temperature range in which TiN is dissolved in some cases, and as the requirements for the weld zone low-temperature toughness become more stringent, the effect described above is less likely to be obtained.
On the other hand, Patent Literatures 1 and 2 each disclose a technique in which, by dispersing fine grains in a steel by means of combined addition of rare-earth elements (REM) and Ti, grain growth of austenite is suppressed, and thereby, the weld zone toughness is improved.
In addition, a technique of dispersing oxides of Ti, a technique of Combining capability of ferrite nucleation of BN with oxide dispersion, and a technique of enhancing toughness by controlling sulfide morphology by means of addition of Ca and REM have also been proposed.
However, these techniques are intended for steels having relatively low strength and low contents of alloy elements. In the case of steels having higher strength and high contents of alloy elements, the HAZ structure is caused not to contain ferrite, and thus the techniques are not applicable.
Accordingly, as a technique that facilitates formation of ferrite in the weld heat-affected zone, Patent Literature 3 discloses a technique in which mainly the amount of Mn added is increased to 2% or more. However, in a continuous cast steel, Mn is likely to be segregated in the central portion of a slab, and the center segregation area ratio increases not only in the base metal but also in the weld heat-affected zone. The center segregation area serves as the origin of the fracture, thus resulting in degradation in toughness of the base metal and HAZ.
On the other hand, in the dual phase re-heating area, carbon is concentrated in the region reverse-transformed into austenite by dual phase re-heating, and the brittle bainite structure including island martensite is formed during cooling, resulting in degradation in toughness. Accordingly, techniques have been disclosed in which, by decreasing the contents of C and Si in a steel composition, formation of island martensite is suppressed and toughness is improved, and by adding Cu, the strength of the base metal is ensured (for example, Patent Literatures 4 and 5). In these techniques, the strength is enhanced by precipitation of Cu by means of aging treatment. However, since a large amount of Cu is added, hot ductility is degraded, and productivity is impaired.